It shows up in tiny type on the third page of your monthly PG&E bill. It's hardly a user-friendly term; it sounds like bureaucratic mumble-jumble.

But now your "baseline usage" will determine how deep you have to dig into your pocket when electricity rate hikes kick in.

Under the tiered rate increase being considered by the California Public Utilities Commission, residential customers who use the least amount of energy -- less than 130 percent of baseline -- will be exempt from rate hikes. Households that use between 130 percent and 200 percent of baseline will see their monthly electricity bills increase 9 percent; those who consume above 200 percent of baseline will pay 36 percent more.

(A separate, 9 percent "temporary" rate hike first implemented in January, and now made permanent by the PUC, is expected to be across the board for residential customers.)

Baseline is supposed to represent the amount of power necessary for essential use. It varies based on climate and time of year, as well as whether your heat is electric or gas.

If you look at page 3 of your PG&E bill, you'll see that your "baseline usage" is billed at a lower rate than your "over baseline usage." Those rates are set by the PUC. The idea behind baseline, which PG&E has used since 1984, is to encourage energy conservation.

Essentially, your baseline allocation is 50 to 60 percent of the average residential consumption in your geographic region. There are 10 such regions in PG&E's territory, and each has its own microclimate.

In hot areas, baseline goes up in the summer when people crank up the air conditioning and down in the winter. In more temperate climates, such as San Francisco, the reverse is true.

For example, in Marin the summer baseline is 231 kilowatt hours per month; in winter it is 267 kwh a month. Statewide, the average monthly usage is 500 kwh per month.

Baseline doesn't take into account the size of your residence or how many people live there. A single person in a studio in Concord has the same baseline allocation as a six-person family in a sprawling house in Walnut Creek.

"It's not perfectly equitable," said Nettie Hoge, executive director of the

Utility Reform Network (TURN), a consumer watchdog group. "The idea is with a broad brush to say there are necessary electrons, then electrons that are beyond that."

There's no easy way to look at your baseline allocation and figure out just how many lights and appliances it includes. It is expressed in kilowatt hours per month. A 100- watt light bulb that is on for 100 hours would use

10 kilowatt hours of electricity. But many other factors go into your energy use -- how old your refrigerator is, how many TVs and computers you have, and, as Hoge said, "Whether you have a teenage son who opens the fridge 55,000 times a day and stares at the milk for 20 minutes."

According to the PUC, 48 percent of the households PG&E serves -- or 1.6 million -- consume less than 130 percent of baseline and thus won't get socked with the newest rate increase.

One-quarter of PG&E customers, or 873,000 households, are medium users who consume between 130 percent to 200 percent of baseline. Those users currently pay an average $69 per month for electricity; under the proposed rate increase that would increase 9 percent, or $6 a month.

The remaining 26 percent of users, or 908,000, use more than 200 percent of baseline. They currently pay an average of $132 a month; that would increase 36 percent, adding an average $47 per household.

How the Proposed Price Hike Might Affect Your Bill

The Public Utilities Commission is considering a proposal to increase power rates for PG&E customers who use more than 130 percent of their "baseline" amount of power, in addition to the 9 percent across-the-board increase approved in January. ÐHere's how you can apply the proposed price increases to your own PG&E bill:

STEP Find your "baseline quantity."

PG&E customers pay different amounts for power based on where they live. Each area has a predetermined baseline - the "reasonable" amount of power needed for that area Ð- and customers pay more if they use more than that amount. First, locate your zone on the map, then use the charts at bottom right to find your baseline quantity and rate tiers. The sample bill at right is for Zone T, with a winter base-line quantity of 267 kilowatt-hours per month. (Summer baseline quantities take effect starting May 1.) These sample amounts are highlighted on chart and in calculations below.

STEP Determine your billing cycle. Baseline quantities in the charts are averages based on a 30-day billing cycle.

Using the sample bill, the baseline quantity of 258.1 kilowatt hours is divided by the billing cycle of 29 days to reach a daily baseline allowance of 8.9 kwh. Multiplying that number by 30 produces an average monthly baseline of 267, corresponding with Zone T for winter electricity customers.

STEP Figure the new cost of your monthly bill.

The proposed plan would charge higher rates for each tier above the baseline allowance, according to the chart at right. Follow the steps below to estimate your costs under the proposed plan. (The example calculations shown here are for a Zone T household using 550 kilowatt-hours per month, slightly higher than the average household usage of about 500 kwh per month.)

-- Multiply usage up to your baseline, in this case 267 kwh, by 12.1 cents. If you have no usage above the baseline, stop. This would be your total electricity usage charge.

Add up amounts to find total cost. A usage of 550 kwh in this zone during the winter would yield a total monthly electricity cost of $81.97.

Baseline quantities in the charts are averages based on a 30-day billing cycle. The baseline quantity shown on your bill may not match the chart if your billing cycle is different than 30 days (as with the sample bill shown here). Using the sample bill, the baseline quantity of 258.1 kilowatt-hours is divided by the billing cycle of 29 days to reach a daily baseline allowance of 8.9 kwh. Multiplying that number by 30 produces an average monthly baseline of 267, corresponding with Zone T for winter electric/gas customers.